Fix overflow detection in unsigned multiplication

Previously the check was done as

	unsigned int a, b, c;
	c = a * b;
	if (c < a)
		// error

but it is wrong, since it catches only a subset of all the
possible overflows.
This patch improves the check as

	unsigned int a, b, c;
	if ((a > 1) && (b > 1){
		if ((UINT_MAX/a) < b)
			// error
	}

An additional case has been added to the tests, with two 32-bit
values that cause overflow and are not detected by the old check.

It is also necessary to break the lowering procedure in case the current
BasicBlock is splitted; in this case it was necessary in order not to
trigger the division by 0 error.